(Maybe) re-add bytes(obj) API

[pipermerriam]

Maybe re-add bytes(obj) API since @carver is so convincing.

[carver]

bytes(PrivateKey) would be especially great.

I would kind of expect bytes(PublicKey) to be supported, by symmetry, but it also seems like something that won't be used much.

bytes(signature) is unusable in a transaction/message signing context, and probably adds more confusion than it helps. But if you want to leave it in, I don't have an objection.